1. Field of the Invention
The present invention relates to a galvanometer for a laser scanner used for laser marking, drilling of fine holes or the like.
2. Description of the Related Art
Various conventional proposals have been made for a capacitive position detector of a galvanometer, as in, for example, U.S. Pat. No. 5,537,109.
FIG. 5A and FIG. 5B are a perspective view and a side view, respectively, explaining a configuration of an electrode section of the conventional art mentioned above.
Butterfly-shaped intervening member 31 with a thickness t, made of a dielectric material having a high dielectric constant or permitivity such as a ceramic, is inserted in an air gap d between a fixed common electrode 30 and a fixed four-division electrode 32. This dielectric having a high dielectric constant intervening member 31 is fixed to a rotatable shaft 33, and air gaps xcex41 and xcex42 are provided between the intervening member 31 and each of the common electrode 30 and the four-division electrode 32, respectively. As the shaft 33 rotates, the change in capacitance between both the electrodes 30 and 32 due to the rotation of intervening member 31 is detected.
Generally, capacitive position detectors of this configuration have a 1.0 mm thick ceramic with a relative dielectric constant of about 6 to 7 as the dielectric having a high dielectric constant intervening member 31, and are designed to have air gaps xcex41 and xcex42 of about 0.1 mm. These detectors have an advantage in that high precision is not required for the parallelism between the electrodes 30 and 32, between the intervening member 31 and each of the electrodes 30 and 32, and for each of the air gaps xcex41, and xcex42, and d, since the air gap d between the electrodes 30 and 32 is large. Another advantage is that the change in detected capacitance due to dimensional changes in an air gap d because of temperature variations is small since the air gap d between the electrodes 30 and 32 is large.
However, even if a dielectric material having a high dielectric constant is used for butterfly-shaped intervening member 31, the capacitance is extremely small at about 2 to 3 pF since the air gap d between the electrodes 30 and 32 is wide, so that a high-frequency of about 500 kHz and a high-voltage of about 500 V signal needs to be applied to a circuit configuration for detecting the change in capacitance accompanied by a change in position. Therefore, extra measures are necessary to overcome noise and in view of the withstand voltage.
Moreover, for the dielectric having a high dielectric constant intervening member 31, ceramic is suitable and is used in practise. However, ceramic is porous and moisture penetrates into the pores under high humidity, so that it has the characteristic of decreased dielectric constant and has a drawback in that errors in detecting the capacitance are caused by the humidity.
The configuration of the conventional capacitive position detector thus has advantages in that machining precision is not required and changes in temperature hardly affect the conventional position detector. However, the conventional position detector also has a drawback in that it has smaller capacitance and it is likely to be affected by humidity.
Accordingly, it is an object of the present invention to provide a moving magnet type galvanometer which has increased capacitance while making use of the above-described advantages, and which is hardly influenced by humidity.
In order to achieve the above object, a moving magnet type galvanometer according to the present invention includes a case; a stator having a ferromagnetic outer yoke held in the case and a coil fixed inside the outer yoke; a rotor having a cylindrical permanent magnet and a front shaft and a rear shaft supporting the permanent magnet; a butterfly-shaped common electrode prepared by patterning a conductive thin film on a surface of a glass disc, the butterfly-shaped common electrode having a flat portion; a hub having a flat disc portion and a hub portion, mounted on a rear end of the shaft through a hole provided in the center of the hub portion and holding the butterfly-shaped common electrode with the flat portion perpendicular to the shaft; a spacer; and a four-division electrode mounted on the spacer so as to oppose the common electrode with an extremely small air gap therebetween. The spacer is mounted on the case so as to provide the air gap in a predetermined dimension.
In this aspect of the present invention, the hub may have a groove for adhesive collection on the surface of the disk portion and the glass disc of the common electrode may have holes for adhesive injection in a section having no conductive thin film; wherein the holes for adhesive injection are injected with an adhesive so as to fix the common electrode to the disc portion of the hub.
Moreover, in the aspect of the present invention, the common electrode may have a conductive thin film pattern formed by etching, after a conductive thin film is deposited or sputtered on the glass disc having a through-hole at its center. The through-hole is fixed, by soldering or conductive adhesive, with a lead pull-out terminal to be connected to the pattern.
In order to achieve the above object, another moving magnet type galvanometer according to the present invention includes a case; a stator having a ferromagnetic outer yoke held in the case and a coil fixed inside the outer yoke; a rotor having a cylindrical permanent magnet and a front shaft and a rear shaft supporting the permanent magnet; an inner race and an outer race of a rear bearing, into which the rotor inside the coil is inserted which supports the rear shaft, the races being fixed to a periphery of the rear shaft and the case, respectively, and an inner race of a front bearing supporting the front shaft, the inner race being fixed to a periphery of the front shaft, and an outer race of the front bearing, the outer race being movable in an axial direction by applying force in the rear shaft direction with springs; a butterfly-shaped common electrode prepared by patterning a conductive thin film on a surface of a glass disc, the butterfly-shaped common electrode having a flat portion; a hub having a flat disc portion and a hub portion, mounted on an end of the rear shaft through a hole provided in the center of the hub portion and holding the butterfly-shaped common electrode with the flat portion perpendicular to the shaft; and a four-division electrode mounted on a spacer so as to oppose the common electrode with an extremely small air gap therebetween. The spacer is mounted on the case so as to provide the air gap of a predetermined dimension. The rear shaft, the hub, and the spacer are made of identical material.
In this aspect of the present invention, the hub, the rear shaft and the spacer may be made of steel or a stainless steel material.
In this aspect, the capacitance increases and humidity hardly has an impact on the galvanometer, in addition to the advantages in that machining precision is not required and that the device is hardly influenced by temperature variations.
The present invention, unlike the conventional art, does not have a configuration for detecting the change in capacitance between both electrodes caused by an angle of a dielectric having a high dielectric constant butterfly-shaped intervening member which is fixed to a shaft in an air gap d between a common electrode and a four-division electrode.
The present invention is configured to detect the capacitance between a common electrode and a four-division electrode by fixing a conductive thin film patterned in a butterfly shape on a surface of a glass disc as the common electrode to a hub fixed to a shaft, and by opposing the four-division electrode thereto so as to maintain parallelism with the common electrode and to maintain an extremely close air gap xcex4(0.04 to 0.05 mm) therebetween.
Accordingly, the capacitance may be increased significantly to about 10 pF, so that a signal of around 70 kHz and 30 V can be applied to a circuit configuration, which is extremely advantageous against noise and in view of the withstand voltage.
Also, ceramic or the like providing adverse humidity effects is not used in the preferred embodiment of our invention.
However, since the air gap xcex4 is small at 0.04 to 0.05 mm,, the capacitance is likely to be erroneously detected by the change in the air gap xcex4 due to thermal expansion as described below. Moreover, the air gap xcex4 has to be kept at 0.04 to 0.05 mm with high precision.
A differential rotation capacity-type angle converter proposed by the present applicant (Japanese Unexamined Patent Publication No. 11-304411) configures a circuit which cancels the change in xcex4 due to thermal expansion by an electric circuit technique, and this proposal is significantly effective in preventing the impact of temperature variations. However, the present invention is designed to minimize the temperature variation in the air gap xcex4 itself.
An outer race of a front bearing is pressed by springs with an appropriate pressure so as to be slidable in an axial direction, and an outer race and in inner race of a rear bearing are fixed to a case and a rear shaft, respectively, by pressing or attaching. The difference in axial dimensional changes between the case and the shaft due to temperature variations is roughly 0.02 mm at 20xc2x0 under the design conditions but is corrected by a 0.02 mm spring displacement, thus providing neither thermal expansion effects nor stress at the rear shaft side.
When the present invention is configured as mentioned above and the rear shaft, the hub, and the spacer described below are also made of a material with an identical coefficient of thermal expansion, dimensional displacement due to thermal expansion will not occur, and an air gap xcex4 between both electrodes will not change even with temperature variations.
If the differential rotation capacity-type angle converter is used along with the present invention, a galvanometer with excellent temperature characteristics will be provided.
Although there still is a problem of thermal expansion of the glass disc having the patterned common electrode, coefficients of thermal expansion of blue plate glass and stainless steel, for example, are 87xc3x9710xe2x88x927 and 100xc3x9710xe2x88x927, respectively, and the difference thereof is small. Considering that the glass thickness is roughly 0.55 mm, the difference may be ignored.
Moreover, a spacer is used to add precision to an extremely small air gap xcex4 of 0.04 to 0.05 mm between both electrodes.
Recently, the machining precision of NC (numerical control) machine tools and the like has improved greatly; however, even components with improved machining precision hardly provide a precise air gap xcex4 since the machining errors of each component accumulate as a result of assembling.
Therefore, it is highly beneficial if a distance L from an end X of the case (see FIG. 1) to a surface of the common electrode is measured for each individual product during assembly and, based on the measurement, a dimension is set by polishing both ends of the spacer.